1. Field of the Invention
The present invention relates to a thermoplastic resin composition. More particularly, it relates to a composite type thermoplastic resin composition which comprises polyamide, a specific graft polymer and a specific copolymer modified with an unsaturated carboxylic acid and has well balanced impact strength (namely notched izod impact strength), moldability and tensile strength, and weld strength as well as appearance.
2. Description of the Prior Art
Polyamide resins are excellent in moldability, heat resistance, mechanical strength, chemical resistance and wear resistance and widely used as materials of, for example, machine parts, electric and electrical parts and automobile parts. However, they suffer from decrease of impact strength in a dry state, and dimensional change and/or decrease of tensile strength due to moisture absorption.
As typical impact resistant rubber-modified resins, acrylonitrile-butadiene-styrene (ABS) resins, AES resins comprising ABS resins a rubber component of which is replaced with ethylene-propylene base rubber and AAS resins comprising ABS resins a rubber component of which is replaced with acrylic rubber are known and widely used as materials of automobile parts, electrical equipment parts, business machine parts. Although these resins are excellent in impact strength and dimensional stability, they usually have relatively poor chemical resistance, and a high impact grade which contains the rubber component in an increased content has decreased tensile strength and moldability.
To overcome the above drawbacks, it was proposed to melt mix polyamide with the ABS resin (cf. Japanese Patent Publication No. 23476/1963). However, since the compatibility between polyamide and the ABS resin is inherently bad, a molded article suffers from delamination and provides an article having low impact strength.
It has been proposed to introduce a functional group having reactivity or affinity with polyamide such as carboxylic acid group and an amide group in the ABS resin so as to modify the ABS resin and to increase the compatibility between polyamide and the ABS resin (cf Japanese Patent Kokai Publication Nos. 11159/1979, 32656/1983 and 93745/1983). However, the introduction of such functional group into the ABS resin may result in great decrease of moldability and unsatisfactory improvement of impact strength. Further, an article molded from the mixed resin has less weld strength (strength along a weld-line).